Vacuum concentration and freeze drying are related methods used for the gentle drying or preservation of thermally sensitive materials. This often involves the removal of aqueous media. As can be seen from the state diagram of water, which can also serve as an example for other solvents, above a certain process pressure (in this case 6.1 hPa for H2O) drying occurs from the liquid phase. At pressures below this level, the water temperature drops below 0 °C.
Drying takes place directly - bypassing the liquid phase - from the ice state. The triple point (here H2O: p=6.1 hPa, T=0 °C) represents, to a certain extent, the boundary state between vacuum concentration (drying) and freeze-drying. One observes the simultaneous occurrence of ice, water and steam in the recipient. The following table contains essential characteristics of both processes.
|Criterion||Vacuum concentration||Freeze drying|
|Evaporation temperature range (gentle process?)||–5 °C to + 20 °C||–60 °C to 0 °C|
|Substances||Usually dissolved; residue is a powder or a crystalline substance||Solids (including ceramics, meat, archeological objects,|
bones, plants and books)
|Solvents||Various solvent types (some exotic)||Aqueous solutions, only small quantities orspecial solvents|
|Water/solvent content||Approx. 5% final content||Low residual moisture possible (< 1%)|
|Process time||Minutes to hours||0.5 – 3 days; several weeks in some cases|
Lyophilisation or freeze-drying is the most product-friendly drying method of all. The underlying physical phenomenon of sublimation means a direct transition from the solid to the vapour state, bypassing the liquid aggregate state. The frozen product is thus dried under vacuum without thawing.
Vacuum concentrators use the combination of heat, vacuum and centrifugal force to evaporate liquid samples. The process is used for evaporation, drying, purification and particularly fast concentration. The principle exploits the fact that the sample boils at room temperature and a pressure of a few hPa without being frozen. Nevertheless, the sample is not thermally stressed under vacuum.